1. Field of the Invention
The present invention relates to a projection exposure apparatus, and more particularly to an exposure apparatus in which the diaphragm aperture of a projection optical system is variable.
2. Related Background Art
Reduction projection exposure apparatus are widely used in the manufacture of semiconductor devices of ultrafine patterns, such as LSI or VLSI, and various efforts are being made to achieve transfer of still finer patterns. For coping with such dimensional reduction of the patterns, in addition to the use of exposure light of shorter wavelength, there has been tried an increase in the numerical aperture (N.A.) of the projection optical system, and there has been realized a projection optical system with a numerical aperture exceeding 0.5.
In the actual projection exposure utilizing a projection optical system with such large N.A., it is important to optimize the illuminating conditions. In this connection it is already known, as disclosed in the Japanese Laid-open Utility Model Sho 61-151, to obtain a suitable balance of the resolution and contrast for a predetermined pattern, by the adjustment of so-called .sigma.-value which is the ratio of the N.A. of the illuminating optical system to that of the projection optical system. In the exposure of fine patterns, it is generally known that a larger N.A. of the projection optical system improves the limit resolution but reduces the depth of focus, while a smaller N.S. deteriorates the limit resolution but increases the depth of focus. In the use of a large N.A. which reduces the depth of focus, the focusing condition has to be extremely strict in the exposure of the fine patterns of the reticle into the wafer, and the pattern transfer cannot be made exactly even by a small change in the focus. For this reason it has been proposed to suitably vary the N.A. of the projection optical system.
However, in the conventional apparatus as explained above, the optimum numerical aperture of the projection optical system has to be determined empirically or experimentally for example by test exposures or other experimental for the fine patterns to be projected, the change in N.A. requires considerable labor and time which are unnegligible in the manufacture when various exposure patterns are projected in succession.